A common useage for a Metal Oxide Semiconductor Field Effect Transistor (MOSFET) is as a switch. In such applications, MOSFETs function as a switch between two terminals, the drain and the source, when the proper control voltage is applied to the gate terminal. For applications requiring high current and medium voltage commercially available MOSFETs are of the N-channel, enhancement mode type. The enhancement mode MOSFET requires that the gate be more positive than the source by some voltage, usually called the threshold voltage, in order to open the channel between the drain and the source. The threshold voltage for commercially available MOSFETs is typically one to four volts.
Prior art designs of isolated drive circuits cannot effectively switch high frequency ac signals at high speed, high power, and high switching frequencies. One problem encountered in prior art designs using a fixed control voltage is a distortion of the signal being switched due to modulation of the switched signal by the enhancement voltage signal applied to the gate. In prior art devices which employ an oscillator to drive the MOSFET gate, the signal being switched may pass through the oscillator's coupling transformer or capacitors. This pass through effect becomes more likely as the ratio of the oscillation frequency approaches the frequency of the switched signal and may result in serious damage to the circuit.
Another drawback of prior art designs is that such designs do not provide a method for supplying a fixed enhancement voltage without injecting steady state voltage into the signal being switched. The enhancement voltage may be defined as the bias voltage present from the gate terminal to the source terminal which is used to turn the switch on.
Prior art devices are also subject to distortion of the switching signal due to switching transients. These transients are present in MOSFETs due to capacitance in the gate to source junction. During the operation of turning a MOSFET switch on or off, the gate to source junction appears as a capacitor. In steady state operation, it is only necessary to maintain this charge against very small leakage currents. For high-speed switching, a large current must be passed to the gate.
FIG. 1 shows an example of a MOSFET switching circuit using two enhancement mode MOSFETs and an isolated transformer drive. In the circuit shown in FIG. 1, the gate voltage at node 10 is referenced directly to the source voltage at node 20. Such a design causes a current to flow into the source. When the oscillator 50 is enabled, the resultant current will mix with the signal current. When the oscillator is disabled, the current will flow, or divide and flow, into the circuits connected to both sides of the transistor switch depending on the voltage levels present. This type of current flow is caused by the internal diodes 30 and 40 (shown in dotted line) that are a part of the internal structure of all enhancement mode MOSFETs. A circuit such as is shown in FIG. 1 has size and cost disadvantages. In addition, at higher switching frequencies, some of the oscillator frequency will mix with and modulate the switched signal. This type of modulation can also result if the magnitude of the switched signal is very small in comparison to the oscillation signal. As the oscillator frequency approaches the frequency of the switched signal, major distortion of the switched signal can take place.